The present invention relates to a vehicle steering system and is particularly concerned with such systems in which power assistance for a steering manoeuvre is provided by an hydraulic servomotor (such as a double acting piston and cylinder device) working fluid for which is provided by an electrically driven pump and controlled by a valve which determines the supply of the working fluid to the servomotor in response to a steering input to assist the intended steering manoeuvre. Known steering systems of the kind aforementioned having electrically driven pumps are usually inefficient as providing a continual drain on the electrical energy available during use of the vehicle and it is an object of the present invention to provide a vehicle steering system in which wastage of electrical energy is alleviated.
Examples of power assisted steering systems which may utilise an electrically driven hydraulic pump for power assistance are to be found in our co-pending British patent application No. 8814358 (as this applies to hydraulic systems) and also in our British patent specifications Nos. 2,193,695 and 2,203,394.